1. Field of the Invention
This invention relates to zoom lenses of the rear focus type to be used in video cameras and, more particularly, to zoom lenses of the rear focus type having either or both of a long back focal distance and a long optical distance from the exit pupil.
2. Description of the Related Art
In the field of art of zoom lenses for use in photographic cameras or video cameras, it has been known to provide a type of focusing configuration in which a lens unit in rear of the front or first lens unit is moved to effect focusing. A great many examples using such a so-called rear focus type have been proposed. This is because the rear focus type has merits that, owing to the use of a lens unit of relatively small size and light weight as the focusing lens, consumption of driving power is saved and that rapid focus adjustment becomes possible to perform.
A zoom lens of the rear focus type as such is disclosed in, for example, Japanese Laid-Open Patent Application No. Sho 63-44614, comprising, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power for varying the focal length, a third lens unit of negative refractive power for compensating for the image shift with zooming and a fourth lens unit of positive refractive power, totaling four lens units. In this so-called 4-unit zoom lens, the third lens unit is made axially movable for focusing.
However, this kind of zoom lens has to create a surplus space for the movement of the third lens unit. Therefore, the physical length of the complete lens tends to increase largely. In another Japanese Laid-Open Patent Application No. Sho 58-136012, the zooming section is constructed with three or more lens units, one of which is made movable for focusing.
In Japanese Laid-Open Patent Application No. Sho 62-247316, a zoom lens comprises, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, the second lens unit being moved to vary the focal length, while simultaneously moving the fourth lens unit to compensate for the image shift. Focusing is performed by moving the fourth lens unit.
In Japanese Laid-Open Patent Application No. Sho 58-160913, a zoom lens comprises, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, totaling four lens units, the first and second lens units being moved to vary the focal length, while simultaneously moving the fourth lens unit to compensate for the image shift. Of these lens units, one or two or more ones are moved to effect focusing.
In Japanese Laid-Open Patent Application No. Sho 62-24213, a zoom lens comprises, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, totaling four lens units, the second lens unit being moved to vary the focal length and the fourth lens unit being moved to compensate for the image shift and to effect focusing.
In Japanese Laid-Open Patent Application No. Sho 63-278013, a zoom lens comprises, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of negative refractive power and a fourth lens unit of positive refractive power, the second lens unit being moved to vary the focal length, and the fourth lens unit being moved to compensate for the image shift and to effect focusing.
In this form of zoom lenses, however, the diverging light beam from the second lens unit is further diverged in passing through the third lens unit. Therefore, the size of the fourth lens unit is caused to increase largely. Moreover, the range of variation of aberrations with focusing tends to increase.
Japanese Laid-Open Patent Application No. Sho 6329718 discloses a zoom lens comprising, from front to rear, a first lens unit made up from a negative lens, a positive lens and a positive lens, totaling three lenses, and having a positive overall focal length, a second lens unit made up from a negative lens, a negative lens and a positive lens, totaling three lenses, having a negative overall focal length and movable for zooming mainly to govern variation of the focal length, a third lens unit made up from one or two or three lenses, having a positive overall focal length, always stationary, playing a role of an afocal one in respect to the angle of emergence, and including an aspheric surface, and, after a little long air spacing, a fourth lens unit made up either from a negative lens, a positive lens and a positive lens, or from a positive lens, a positive lens and a negative lens, totaling three lenses, having a positive overall focal length, movable for maintaining the constant focal plane during zooming, that is, playing a role of a so-called compensator, and made movable for focusing. However, if the ratio of the focal length of the third lens unit to that of the fourth lens unit is too high, it could be difficult to obtain a sufficiently long back focal distance.
Japanese Laid-Open Patent Application No. Hei 5-72472 discloses an aspherical zoom lens comprising, from front to rear, a fixed first lens unit of positive refractive power, a second lens unit of negative refractive power for varying the focal length, a fixed third lens unit of positive refractive power having a light collecting action, and a fourth lens unit of positive refractive power axially movable for maintaining the constant image plane. This zoom lens has its third and fourth lens units spaced apart by a relatively long distance. The first lens unit is comprised of, from front to rear, a negative lens, bi-convex lens and a positive meniscus lens. The second lens unit is comprised of a negative meniscus lens, a bi-concave lens and a positive lens. The third lens unit is made up from a single lens of which at least one surface is aspherical. The fourth lens unit includes at least one aspherical lens and is made up from one negative lens and two positive lenses.
However, this form of zoom lens, though having a long back focal distance, seems, in view of the embodiments, that when zooming to a middle position, the distance from the exit pupil to the color separation optical system becomes considerably short, causing color shading to occur in the color separation optical system with a high possibility.
In U.S. Pat. No. 4,299,454, a zoom lens comprises, from front to rear, a positive first lens unit, a negative second lens unit and a positive rear lens unit, wherein at least two lens units including the negative second lens unit are moved to effect zooming, and the negative second lens unit is made up from first and second negative lenses and a positive doublet. However, the embodiment teaches that the third lens unit has its frontmost lens made negative in power. In this case, the third lens unit is selected to be movable. Therefore, the operating mechanism tends to be complicated. So, it is difficult to obtain a sufficiently long back focal distance.
U.S. Reissue Pat. No. 32,923 discloses a zoom lens comprising, from front to rear, a positive first lens unit, a negative second lens unit, a stop, a positive third lens unit and a positive fourth lens unit, the first and fourth lens units moving in the same direction during zooming, and the stop remaining stationary during zooming. The embodiments teach that the third lens unit has its frontmost lens made negative in power. However, it cannot be said that the back focal distance is long enough.
In general, the use of the rear focus type in the zoom lens, as described before, gives a configuration having the merits that the lens system is reduced in size as a whole and also that rapid focusing becomes possible and further that short focusing is easy to do.
For its counterpart, however, the range of variation of aberrations with focusing is liable to increase largely. So, it becomes very difficult to assure minimization of the bulk and size of the entire lens system in such a manner that good stability of optical performance is maintained at a high level throughout the entire focusing range.
It should also be noted that, at present, the single chip type CCD is widely used in video cameras for home use. In this case, there is no need to use a color separation prism and associated parts therewith which are prerequisite for the multiple chip type CCD used mainly in the professional video cameras. The zoom lenses for video cameras for home use have, therefore, their back focal distance and eye relief made relatively short.
In application to the video cameras using the multiple chip type CCD, however, because the color separation prism and associated parts therewith must be arranged in rear of the zoom lens, the zoom lens suffers a problem that, as compared with the zoom lens for the video camera using the single chip type CCD, the back focal distance has to be relatively long, and the eye relief has to be sufficiently long.